Phosphoric acid is formed in fertilizer production as an intermediate product useful in the manufacture of various end products, such as ammonium phosphate, triple superphosphate, and various solid and liquid mixed fertilizers. Phosphoric acid itself may be produced by the reaction of phosphate rock with sulfuric acid. During such a reaction, a by-product of calcium sulfate, either as gypsum, or a hemihydrate, is generated along with the phosphoric acid. Typically, about 2-3 tons of sulfuric acid, or the equivalent of nearly 1 ton of sulfur, are required to produce phosphoric acid having one ton of soluble P.sub.2 O.sub.5 with concurrent production of about 6 tons of the by-product calcium sulfate, wet phosphogypsum.
The current practice of using sulfuric acid as a phosphate rock acidulant has a number of disadvantages. One major disadvantage is a dependence of the fertilizer industry on reasonably priced and readily available sulfur which is used in production of the sulfuric acid with which the phosphate rock is reacted. Sulfur production by the well known Frasch process is energy intensive and sulfur recovered by this process increases as fuel prices escalate. Higher energy prices also have resulted in the closing of older, less efficient sulfur mines, reducing the availability of domestically produced Frasch sulfur. Also, the known reserves of elemental sulfur suitable for Frasch mining are being depleted. Other sources of sulfur, such as sour natural gas, exist and may be utilized to some extent subject to price and availability.
A second disadvantage of the use of sulfuric acid for phosphate rock acidulation is the production of a large tonnage of calcium sulfate by-product, as noted above, usually in the form of gypsum containing residual acidity and other impurities. Although this by-product might be further processed to a useful form, the processing is practical only in a few geographical locations having uncommon economic conditions which justify the relatively large costs of producing useful products. Thus, the potential value of the gypsum may not generally be realized and its disposal presents a significant pollution control problem.
There are also advantages to recycling the sulfur normally discarded in the by-product calcium sulfate and several processes have been developed for this purpose. One such process, for example, is the Chemie Linz Gypsum-Sulfuric Acid Process. In this process, a mixture of calcium sulfate, sand, clay, flue ash and coke is fed to a rotary kiln, which may be fired using coal dust, fuel oil or natural gas. The clinker produced by the kiln is cooled and ground to cement. Sulfur dioxide-containing gas leaving the kiln is then cleaned, and a small amount of air is added in order to effect the oxidation of sulfur dioxide to sulfur trioxide in a catalytic converter at an SO.sub.2 strength of about 6%-7%. The SO.sub.3 thus produced is absorbed in water to produce sulfuric acid. Although such processes have been operated in commercial plants, several disadvantages arise: the quantity of fuel required to form the clinker, the high residence time required in the kiln (60-90 minutes), the co-production of large tonnages of by-product cement which must be sold or otherwise discarded, and the need for a sulfuric acid plant specifically designed to operate with a gas feed containing only 6%-7% SO.sub.2.
Another process produces sulfur dioxide and lime (CaO) from calcium sulfate. Calcium sulfate particles are heated at about 1200.degree. C. in the presence of reducing gases for 1 to 2 hours; the evolved sulfur dioxide is cooled, cleaned, and converted to sulfuric acid through a contact oxidation process followed by reaction with water. Lime is produced as a valuable by-product, but this process has some of the same disadvantages as clinker processes. In addition, the conversion is energy intensive, requiring about 7700 cubic feet of natural gas per ton of H.sub.2 SO.sub.4 produced. Further, the sulfur dioxide feed to the sulfuric acid plant is still dilute (approximately 6%-7%), and thus cannot be used in a conventional plant designed to burn more concentrated sulfur without extensive modification. Finally, by-product calcium sulfate is not a preferred feed material because it is contaminated with fluorides and phosphates. In order to effectively utilize calcium sulfate from the most widely used phosphoric acid processes, it is necessary to remove impurities such as fluorides.
Conventional processes have been developed for the reduction of calcium sulfate to calcium sulfide. The reduction of CaSO.sub.4 to CaS is highly endothermic, requiring large amounts of fuel and high temperatures. One conventional process utilizes natural gas in an amount equivalent to 0.5 moles of carbon per mole of calcium sulfate. Oxygen is injected near the top of the bed in a reactor, forming sulfur dioxide. Calcium carbonate and calcium chloride are by-products. Another process uses coal, oil or natural gas to reduce calcium sulfate at a temperature of about 840.degree. C. The calcium sulfide produced is reacted with water and carbon dioxide using the Chance process to yield hydrogen sulfide. The hydrogen sulfide can then be converted to sulfur through the Claus process, or burned directly in a conventional sulfuric acid plant.
Although the reduction of calcium sulfate to calcium sulfide proceeds more rapidly in the above process than the following process types, generally a reaction time of an hour or more is required to achieve reasonable conversion. In addition, the gypsum must first be heated for dehydration before being subjected to the reduction step.
As calcium sulfate can be reduced to calcium sulfide, other sulfates can be reduced to their corresponding sulfides. For many years most sodium sulfide was made by the reduction of salt-cake (sodium sulfate) using coal or coke. This reduction was carried out in furnaces at temperatures of over 980.degree. C. Typically, 0.4 to 0.45 parts of reducing coal were required to reduce 1 part salt-cake. A complete reduction of the salt-cake generally required about 2.5 hours. Since reaction conditions were severe, the furnace brickwork suffered severe wear. Similarly, in yet another process, sodium sulfide was produced by reducing salt-cake with hydrogen at about 800.degree. C. in brick-lined, insulated rotating kilns. Hydrogen was circulated through the kilns at a rate equal to 7 times the rate of hydrogen consumption.
Several of the processes outlined above for sulfate reduction by using coal also result in the formation of by-product gases. It should be noted that various alkali, alkaline earth and transition metal compounds have been found to be effective catalysts in the coal and coal char gasification reactions at the high temperatures discussed above. These effects may be important in reactions both on the coal surface and in the gas phase. The alkali metal compounds are effective catalysts for both the char-steam and the char-carbon dioxide reactions, while the alkaline earth metals such as calcium, are effective for the char-carbon dioxide reaction. Of the alkali metals, sodium and potassium have been found to be most effective catalysts if they are added in the form of carbonates; they are least effective as phosphates.
It has been observed that the activity of these materials increases as the amount of catalyst increases up to a "saturation point". Thus, it has been found that solutions of sodium or potassium carbonate catalyze the char-steam reaction at about 700.degree. C. The rate of gasification was found to be roughly proportional to the concentration of potassium up to 15% based on the carbon content of the coal.
Notwithstanding the above technologies, there exists a need to convert the production of environmentally troublesome by-products, such as calcium sulfate, to useful materials or at least environmentally neutral materials.
This and other problems are overcome by the present invention wherein an improved process is provided for gasifying a carbon species and recoverying sulfur from a reduced sulfate while producing environmentally satisfactory results.